1. Technical Field
The present invention relates to an image forming apparatus in which an excellent image is formed without a density irregularity by using a simple circuit and a method of forming an image.
2. Related Art
Generally, an electrophotographic toner image forming unit includes a photoreceptor that serves as an image carrier having a photosensitive layer formed on an outer circumferential surface, a charging unit that uniformly charges the outer circumferential surface of the photoreceptor, an exposure unit that selectively exposures the outer circumferential surface uniformly charged by the charging unit so as to form an electrostatic latent image, and a developing unit that applies a toner serving as a developer to the electrostatic latent image formed by the exposure unit so as to form a visible image (toner image).
In a tandem-type image forming apparatus that forms a color image, the plurality of (for example, four) toner image forming units are disposed with respect to an intermediate transfer belt. As another image forming apparatus, there is an intermediate transfer belt type image forming apparatus in which toner images formed on a photoreceptor by a single color toner image forming unit are sequentially transferred to the intermediate transfer belt, a plurality of colors of toner images (for example, toner images of yellow, cyan, magenta, and black) overlap on the intermediate transfer belt, and a color image is obtained on the intermediate transfer belt.
In the tandem-type image forming apparatus that has the above-described structure, it has been generally known that an LED or an organic EL element serving as a light emitting element is used in a line head. In a light writing line head where the LED or the like is used as a light source, light amounts of a plurality of light sources (light-emitting units) are not uniform. In this state, if a write operation is performed, a density difference (striation) according to the light amounts, that is, a density irregularity may occur in the image formed by the write operation.
In order to prevent the density irregularity from occurring, according to the related art, a circuit is provided in which a light amount of each of a plurality of light sources that are provided to correspond to respective pixels is corrected at the time of a write operation to make a density uniform. The light amount is corrected by changing a driving current or a lightening time of each light source. Specifically, in order to correct the light amount, a light amount of each light source is measured at the time of a shipment of a line head, a correction value of a lighting time or a driving current corresponding to each pixel is written in a memory incorporated in the line head, and when the correction value is used, that is, when an image write operation is performed, the correction value is read and the driving current or the lightening time of each light source is corrected.
However, the correction circuit that is generally used needs to be provided to correspond to each pixel independently from a lightening control of each pixel. In particular, when performing a grayscale control that varies a light intensity of each pixel according to a density of an image, the grayscale control and the light amount correction need to be performed independently from each other, which complicates a circuit. Further, in a recent line head, with an advance of a light amount correcting technology, a light amount of each pixel on a scanned surface can be corrected such that the light amount becomes approximately uniform, that is, the light amount can be corrected within an error range of about 1%. However, in an actual image, a longitudinal streak/stripe may occur in a direction where a scanned medium moves. This is due to a case where even when a light amount is uniform, a size or shape of an image forming spot is different due to an irregularity of an image forming characteristic according to the location of a gradient refractive index type (rod) lens array (product name ‘Selfoc lens array (SLA)’, manufactured by Nippon Sheet Glass Co., Ltd.) which is generally used when an image is formed on a scanned surface by a light beam of each pixel. As such, a light amount of each pixel is corrected together with an irregularity in an image forming characteristic for each pixel due to the SLA. The term “image forming characteristic” in this application means the characteristic of the image of the light source formed by said lens array and does not means the pictorial image (toner image) characteristic formed by image forming apparatus which is presented on this appreciation.
In particular, in recent years, the line head is often used in an electrophotographic color page printer. In a color image, as compared with a monochromatic image, it is required for photography or a graphic to be further improved in expression and reproducibility, and a precision light amount correction needs to be performed. Meanwhile, the light amount correction is digitally performed. However, since the correction value needs a large amount of information, that is, a large number of bits in order to perform a precise light amount correction, it is likely for a light amount correcting circuit to be large-scaled.
In order to solve the above-described problems, various technological methods have been suggested. For example, JP-A-05-270057 discloses a method in which a correction value of each pixel is determined on the basis of a density difference of an actual image, not a light amount of a pixel. Further, JP-A-2002-292922 discloses a method in which a correction value is changed according to a light amount (grayscale value) of a light emitting unit. Furthermore, JP-A-03-190765 discloses a method in which a grayscale value and a correction value of each pixel are independently controlled. Furthermore, JP-A-08-142406 discloses a method in which a light amount is corrected on the basis of a width at any section in an intensity distribution of an image forming spot of each pixel. Furthermore, JP-A-2004-148661 discloses a method in which a correction value determined by an area of a beam (spot) is modified to a correction value according to an angle of a screen for performing a grayscale expression. Furthermore, JP-A-2004-188855 discloses a method in which correction data of each pixel is determined on the basis of characteristic data calculated by a beam area, a beam diameter, an MTF, or the like of each pixel and light amount data indicating a light amount of each pixel.
JP-A-05-270057 discloses a method in which a correction value is determined on the basis of a printed result, that is, a density irregularity but a correction operation is performed by controlling a light amount of a light emitting unit in a line head. Since a density of an image where an evaluation of a density irregularity is performed or a degree to which a density irregularity occurs due to a grayscale screen is different, this method is only effective in a specific image. Further, a method in which with respect to an individual line head, printing is actually performed and a correction value is calculated needs a complicated process. Therefore, the method cannot resolve the above-described problems.
A degree to which a density irregularity occurs due to an irregularity in a light amount of a light emitting unit is different according to a density of an image to be expressed. When the density is extremely high, that is, the density enters an approximately saturation state, an influence due to an irregularity in a light amount of a light emitting unit is a little. Further, even in a portion where a density is low, that is, a highlight portion, an influence due to an irregularity in a light amount of a light emitting unit is a little. Meanwhile, a portion where a density is middle may be easily affected by an irregularity in a light amount of a light emitting unit, and a streak-shaped density irregularity may easily occur.
According to the method disclosed in JP-A-2002-292922, in order to solve the above-described problems, a correction value is varied according to a grayscale value of each light emitting unit (pixel). However, in most of image forming apparatuses, since a plurality of pixels are disposed as a dot-shaped or line-shaped screen and a density is expressed, a grayscale value of an individual pixel is not in proportion to a grayscale value of an actual image. For this reason, even when using the method disclosed in JP-A-2002-292922, its effect is limited to a specific case.
The correction circuit that is disclosed in JP-A-03-190765 needs to be provided to correspond to each pixel independently from a lightening control of each pixel. In particular, when performing a grayscale control for changing a light intensity of each pixel according to a density of an image, a grayscale control and a light amount correction need to be performed independently from each other. As a result, the control circuit becomes complicated.
As such, in any one of the above-described methods, a light amount of each pixel becomes a correction target. However, as such, a density irregularity, such as a longitudinal streak, which occurs due to the line head, is varies according to the density of the image, and is varied according to a grayscale screen to be used. In the recent printers, types of a grayscale screen are used in a differentiated manner according to types of the image to be printed. For example, since a natural image, such as a photograph, is needed to have a uniform grayscale more than the resolution of the image, the pitch of dots or fine parallel lines of the grayscale screen to be used (this is referred to as the number of lines and is represented as the number of dots or fine parallel lines per 1 inch=25.4 mm) is decreased. Meanwhile, the characters or the line drawings, and the graphic are not needed to express the slight grayscale variation but need the resolution. Therefore, the screen having a large number of lines is used.
The density irregularity due to the light amount irregularity of the line head or the image forming characteristic irregularity of the SLA may be easily viewed if the number of lines of the grayscale screen to be used is increased. Accordingly, there is a problem in that even if the correction value of each pixel is applied, the degree of the density irregularity is different according to the screen to be used. In terms of this, there is a problem in that even when using the method disclosed in JP-A-08-142406, the correction cannot be effectively performed on the difference between the density irregularities due to the difference between the grayscale screens.
In the method disclosed in JP-A-2004-148661, a degree to which each pixel is corrected is changed according to the angle of the grayscale screen. However, as described above, even when the grayscale screen having the same angle is used, since the longitudinal steak of the image is different according to the darkness of the grayscale to be expressed. Therefore, there is a problem in that even when the correction value is changed by the screen angle, the effect is limited. Further, even when the screen angle is the same, if the screen pitch is different, the influence due to the characteristic irregularity of each pixel is different. In terms of this, an effect of when the correction value is changed by the screen angle is low. In the method disclosed in JP-A-2004-188855, an image forming characteristic is added to the light amount of each pixel, and the correction data is determined. However, in this method, since all of the characteristics of the grayscale screen are not considered, the characteristics of the grayscale screen, or the occurrence of the longitudinal streak due to the density of the image to be expressed is changed, and thus the correction effect is limitative.
Further, even when the angle of the grayscale screen and the number of lines are the same, an influence on the optical characteristic of each pixel due to the density to be expressed is different, and the correction effect may not be achieved. For example, when the image forming characteristic of an arbitrary pixel is degraded more than the other pixels (the image forming characteristics are not sufficient and the spot becomes large), the density becomes smaller in the low density portion, and the density becomes larger in the high density portion. In this case, the density cannot be uniformly corrected from the low density portion to the high density portion with a single correction value. As such, an influence that is applied to the image quality due to the light amount irregularity of each pixel of the line head, in particular, the optical characteristic irregularity is different according to the pitch of the screen or the density of the image. As a result, even when the characteristic of each pixel is evaluated with high precision, it is difficult to achieve a sufficient correction process.